Let's quickly list the pros and cons of a subtractive playstyle.
--Allows for low-energy play; by focusing on abilities that you are good at, you don't need to expend unnecessary mental energy executing them.
--Gives you concrete, measurable improvement.
--You quickly become good at some aspect of the game.
--It can be difficult to learn which skills are worth improving and focusing on. How do I know which skill I should be putting my focus on, and which ones aren't worth my time right now?
--You may wind up with a gimmicky style and stagnate as a player.
--You may become easy to read.
--If you practice against people who aren't very good, you may think you've mastered a skill but really haven't, and this comes back to haunt you later.
--If you play against somebody who is better with your specialties than you are, or they have mastered a counter to your specialties, then you have nothing to fall back on.
The pros are great, and don't really need elaborating on. Instead, let's look at the cons and find ways to counteract them. After all, I managed to list more of them than the pros, which kind of makes you rethink whether this is a worthwhile endeavor.
Problem: It can be difficult to learn which skills are worth improving and focusing on.
Solution: Study higher level players and pick important, successful parts of their game to emulate. It's tempting to spend time mastering neat tricks that look really cool but have no use or application. I used to devote a lot of time to practicing things like triple shine into short hop reverse double laser; it goes without saying that I've never used it to any effect in an actual game. I should have been watching videos of the top players and seeing which aspects of their game were successful. If they use some move as their bread and butter approach, pay close attention to how they use that move and try to use it in the same way. Learn everything you can about that one move and why it works so well for them, then master it.
Think critically, and focus on subtle details. For instance, look at JMan and his use of Fox's n-air, like in this video: http://www.youtube.com/watch?v=NMAu3XYdVk0 Look at about 0:14, when JMan starts n-airing Scar from the right side of Dreamland across the stage. The first n-air comes out almost the very frame JMan leaves the ground, and he fast falls as soon as possible. He does it again with the second one; JMan is n-airing as fast as he possibly can. However, after jabbing Scar off the ground, he uses a third n-air that is NOT as fast as the first two. This is because if he had n-aired sooner, he'd have hit Scar's shield too early and risked getting shield grabbed; delaying it put him as close to the ground as possible . I'm willing to bet JMan quickly recognized that; he understands how to use n-air properly even when the situation changes in a subtle way. Sure enough though, after he shines against Scar's shield he throws out a fourth very fast n-air to prevent Scar from escaping his shield pressure.
So on the one hand, you may watch JMan and think "if I can n-air as fast as he does, I'll be a top player," which is incorrect. You should be able to control the n-air as well as JMan, and understand its situational uses as well as JMan, and then... you will be able to n-air as well as JMan, and that's it. But it's a great start, and n-air is a great move.
There is a downside to doing this method: with pure imitation you can only become as good as the player you emulate. However, by watching their videos you can also see what sorts of things they get punished for, or where they fail to follow up effectively. That gives you a good roadmap for how you can eventually surpass them. If they're very effective with a character's b-air but they space f-air poorly, you have a clue as to how you can later develop a skill that they don't have.
That brings me to another point: make your own videos and look for some part of your game that gets you into trouble a lot. Focus your energy on improving those parts of your game; if Peach's d-smash destroys you, deliberately let yourself get hit by it and practice teching out of it, practice DI'ing it so you only get hit by it once, learn how to space away from it when Peach CC's you; focus on all the ways that you can get around the destructive power of this one move. When it's no longer an issue for you, train the next trouble spot of the matchup. You may not even need videos if you have a decent memory. Just think of the things that beat you, and turn all your attention to countering one of those things. Once countering it is second nature, move on.
To start with, however, here are some examples of skills that are good to work on and improve: tech chasing with grabs, spacing moves to avoid shield grabs/CCs, dash-dancing away from a move to punish it, sweet-spotting your character's recovery, perfectly timing shield grabs, absolutely mastering your character's chain grabs (if you have one). Lastly, being able to viciously gimp Fox and Falco will always come in handy in tournament, so master that too; pretty much every character can do it :)
Problem: You may wind up with a gimmicky style and begin to stagnate as a player.
Solution: Don't. Resist laziness and keep finding new skills to improve. If you start seeing results and you wind up winning a lot of your games, you may start thinking that your game doesn't need expanding. However, you must remember this: the more mastered skills you have at your fingertips, the more unpredictable, versatile, and creative you can be. Not only that, it often happens that when you learn a new skill it improves the efficacy of your old ones; you get more mix-ups and more follow-ups, which in turn makes you deadlier.
Against some players you can get away with a lame and gimmicky style. Don't let that spoil you into stagnating; the end goal is expanding your game, not limiting it. Remember this, particularly because of the next problem...
Problem: You may become easy to read.
Solution: At first, there really isn't one. Hopefully you'll be *so* good at whatever your technique is--maybe dash-dance camping into a grab, or tech-chasing with u-air on a platform, or something--that the opponent can't avoid it even if he knows it's coming. However, at high levels of play that's very unlikely, because top players are experienced at dealing with many different situations and will probably know the counter to your technique.
Right now though, we're assuming you're NOT at that high level of play, and you're just trying to improve. Don't worry about those top players at this moment. A lot of people have trouble dealing with various tactics; make your technique as unstoppable as it can be, and you'll quickly surpass all those other people. Then you can add new techniques and become *more* unstoppable. As you learn the intricacies of each skill, you end up making better and better decisions, and you can handle more situations.
I reiterate: the only way to avoid the problem of being gimmicky, stagnant, and easily read is to keep mastering new and useful stuff. For now, accept that your game will be limited and move on.
Problem: If you have poor competition, you may think you've mastered a skill when you really haven't.
Solution: Hopefully, you *do* have at least one person in your area as motivated as you. Even if they aren't good, they can still be a sparring partner for training specific situations.
Try this: set a match to time mode and turn the timer off. Pick something to practice, like drill-shining a shield versus shield grabbing. The two of you will be competing to see who is better at the situation, and you just play at it. Even if your opponent isn't good in the context of a real match, the two of you can still improve in this one area, in that one context. Because the timer is off and there is no stock, there's no pressure; you can experiment with the timing, and if you get shield grabbed or shined... so what? You try again, and again, and cement the proper timing into your hands and mind.
You can also try gimmick matches; your opponent will ONLY edge-camp with Sheik and you'll be Fox, and you'll both try to win that scenario again and again. You will only play on Pokemon Stadium during its transformations; when you're not in it, you both just dash dance around and practice movement tricks.
The goal of this is not only improving your skill at these situations, but to make your competition better.
If you *don't* have a training partner or your local scene is far beneath your level, then you'll have to be imaginative. Even when practicing Brawl by himself, Mew2King tried to imagine and construct in-game situations that he could practice. He'd send a computer Snake into the air with Metaknight, then intentionally up-air beneath him and miss, pretending that the computer was a human who would try to fall and air dodge through him. M2K would fall as he u-aired, then jump and n-air, tracking the computer's movement.
If you're alone, try and practice situations that are very likely to occur. For example, you might play as Marth against a computer and throw it onto a platform. Then you practice hitting it with a tipper after it misses its tech. Try to learn where you need to stand to have the most control over the platform, so that when an enemy is stuck above you, you never miss the tipper. Do the same thing with up-tilt and f-tilt, learn the different spacings on Battlefield, Yoshi's Story, and Pokemon Stadium.
Watch videos and practice scenarios as though you were in those matches. Even though no two matches are identical, most situations are remarkably common and you can find ways to rehearse them, even without an opponent.
Problem: If you play against somebody who can counter your specialty, you'll have no contingency plan.
Solution: There isn't one. In a lot of ways, it's not even a problem. You can say "I should have practiced something else," but that's not very helpful; the same problem will still occur, maybe even against the same opponent. You may wish you were better all-around, but if your opponent is experienced that won't help much either. You may wish to plan and be ready for every scenario and every kind of opponent, but that's not realistic. It's highly likely that you will come against some player or some tactic that simply outclasses what you've got at the moment. Understand that these kinds of losses are possible and then use them as maps. Use them to tell you what skill you should train next, or how you can further expand the one you're using.
Here's something to keep in mind though; in the tough matches that matter, you may have to take risks to win. Perhaps you *will* have to do something with a low chance of success because it's the only path to victory, or it's just more likely to succeed than your other options. It may be something that you're bad at, something that's crazy and situational, or even something that's just really really stupid. There's nothing wrong with going for these, provided you believe it's pivotal to winning that particular game. You never want to give up on a match, or completely assume at any moment that it's hopeless for you. I've seen people drastically improve in the middle of a match to come back and beat people they "should have" lost to; in a few rare instances, I've done it myself.
Furthermore, don't let any kind of pride dictate the way you play. You are not forced to win with your chosen style, or finish with a signature combo. You also aren't forced to play in an entertaining way, or mix up your game when it's disadvantageous. Pick the path that you believe will lead you to victory. If it's your specialty, fine. If it's something you've never done before, fine. If it's boring, fine. If it's risky and exciting, fine. Just remember that in a serious match, you are playing to win by whatever means available to you.
Please comment if you have any questions or comments; this section can be organic and I'll add to it if people raise important or interesting points.